Allow to set default value for config options with AsciiDoc attributes in the docs

[wjglerum]

Make it possible to use AsciiDoc attributes directly in the ConfigDocDefault annotation. This is especially useful to pass Maven properties as AsciiDoc attributes to the config properties as a default value for the documentation.

For example, we now define the keycloak.docker.image in a central place, however we lost the default value in the documentation. With this change we can also use AsciiDoc attributes as default value again for the documentation.

This should cover the initial idea from @gsmet, see #51024 (comment)

Locally this rendered the default value again for the Keycloak Docker image

Once integrated we can also do this for the other Dev Services.

[gsmet]

Looks mostly good, thanks a lot. I added a couple of minor comments.

[wjglerum]

Looks mostly good, thanks a lot. I added a couple of minor comments.

Thanks! Processed your feedback, if it looks good I can squash the commits

[github-actions]

🎊 PR Preview 271f56c has been successfully built and deployed to https://quarkus-pr-main-51098-preview.surge.sh/version/main/guides/

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[gsmet]

I added a few comments.

[gsmet]

Also, not sure if you're aware but we can probably also improve things for these (not sure they are all used for Dev Services but at least some of them are):

        <elasticsearch.image>docker.io/elastic/elasticsearch:${elasticsearch-server.version}</elasticsearch.image>
        <logstash.image>docker.io/elastic/logstash:${elasticsearch-server.version}</logstash.image>
        <kibana.image>docker.io/elastic/kibana:${elasticsearch-server.version}</kibana.image>
        <opensearch.image>docker.io/opensearchproject/opensearch:${opensearch-server.version}</opensearch.image>

        <!-- Database images for JDBC/Reactive/Hibernate tests and devservices -->
        <postgres.image>docker.io/postgres:17</postgres.image>
        <mariadb.image>docker.io/mariadb:10.11</mariadb.image>
        <db2.image>icr.io/db2_community/db2:12.1.0.0</db2.image>
        <mssql.image>mcr.microsoft.com/mssql/server:2022-latest</mssql.image>
        <mysql.image>docker.io/mysql:8.4</mysql.image>
        <oracle.image>docker.io/gvenzl/oracle-free:23-slim-faststart</oracle.image>
        <mongo.image>docker.io/mongo:7.0</mongo.image>